Cladistics was introduced by the German entomologist Willi Hennig, who put
forward his ideas in 1950. He wrote in his native language, so these were completely
ignored until 1966 when an English translation of a manuscript was published
under the title “Phylogenetic Systematics” (Hennig 1966). It is
not an easy book to read but fortunately many others have been written that
have both fl eshed out and distorted his ideas. Hennig’s most important
contribution was to offer a precise defi nition of biological relationship and
to suggest how that relationship might be discovered.
Taxon and character relationship
Hennig’s concept of relationship is illustrated in Figure 1. Considering
three taxa, then the salmon and the lizard are more closely related to each
other than either is to the shark. This is so because the salmon and the lizard
share a common ancestor, ‘x’, which lived at time t2 and which
is not shared with the shark or any other taxon. Similarly the shark is more
closely related to a group ‘salmon+lizard’ because the shark,
salmon and lizard together share a unique common ancestor – ‘y’,
which lived at an earlier time t1. The salmon and lizard are called sister-groups;
the shark is the sister-group of the combined group salmon+lizard. By extension,
the lamprey is the sister-group of shark+salmon+lizard. The aim of cladistic
analysis is to discover this sister-group hierarchy, and express the results
in branching diagrams. These diagrams are called cladograms, a reference to
the fact that they purport to express the genealogical units or clades (the
word ‘Cladistics’ was, ironically, coined by Ernst Mayr –
a life-long opponent of cladistic classification). The aim of cladistics is
to search for the sister-group, and the concept of two taxa being more closely
related to each other than either is to a third (the three-taxon statement)
is fundamental to cladistics.
Figure 1. Hennig’s concept of relationships among taxa A – D. See
text for discussion.
Sister-groups are discovered by identifying characters (or character states)
that are uniquely shared by two of the three groups under consideration. But
not just any characters (or character states – we will deal with the relation
between character and character state in the next article).Hennig made a distinction
between two types of characters (or character states) and this distinction depended
on where they occurred in the phylogenetic history of a particular group. The
character or the state of the character which occurs in the ancestral morphotype
he called “plesiomorphic” (near to the ancestral morphology),
and the derived character, or the derived state, he called “apomorphic”
(away from the ancestral morphology). Here, it is only necessary to emphasise
that the terms apomorphic and plesiomorphic are relative terms – relative
to a particular systematic problem. In Figure 2A character state “a”
is plesiomorphic and “a prime” is apomorphic. State “a
prime” is presumed to have been present in the ancestral morphotype which
gave rise to taxa B and C. The presence of character “a prime”
– the apomorphic state – in taxa B and C is evidence of their immediate
common ancestry and their sister-group relationship. “a prime”
is a shared apomorphy or a synapomorphy suggesting that taxa B and C are more
closely related to each other than either is to A. In Figure 2B “a prime”
is apomorphic with respect to “a” but it is plesiomorphic with
respect to “a double prime”. So, just as the relationship of taxa
is relative, so is the relationship of characters (or character states).
Figure 2. Hennig’s ideas of relationships between character states.
See text for discussion.
Hennig thought that you could decide which was the apomorphic state and which
was plesiomorphic before you did the analysis. He had several criteria for this
of which stratigraphic order was the most relevant to us – the state of
a character that occurs earlier in the fossil record is to be regarded as the
plesiomorphic state. This did not go down very well with neontologists, nor
with many palaeontologists, because it relied on the faithfulness of the fossil
record to document the truth. Today, there are two criteria that are used: the
outgroup and ontogenetic sequence, both of which we will explore in the next
Hennig introduced a third state that he called autapomorphic. This is the state
that occurs in only one of the taxa under consideration. And once again, autapomorphic
characters in one analysis may be synapomorphies in another.
A real example is given in Figure 3. In Figure 3 characters numbered 3 and
4 are synapomorphies suggesting that the lizard and the salmon shared a unique
common ancestor ‘Z’. It suggests that characters 3 and 4 arose
in ancestor ‘Z’ and were inherited by the salmon and the lizard.
Shared primitive characters (symplesiomorphies) are characters inherited from
a more remote ancestry and are irrelevant to the problem of relationship of
the lizard and the salmon. For example, the shared possession of characters
1 and 2 in the salmon and lizard would not imply that they shared a unique common
ancestor because these attributes are also found in the shark. Characters 1
and 2 may be useful at a more inclusive hierarchical level to suggest common
ancestry at ‘Y’. With respect to the three-taxon problem (shark,
salmon and lizard) then characters 1 and 2 are symplesiomorphies and they suggest
nothing other than that the shark, salmon and lizard are a group. Similarly,
characters 5 – 9 and 10 – 12 are autapomorphies and irrelevant to
discovering relationships since they are each found in only one of the taxa.
Sister-groups are discovered by identifying shared derived apomorphic characters
(synapomorphies) inferred to have originated in the latest common ancestor and
shared by descendants. These synapomorphies can be thought of as evolutionary
homologies: that is, as structures inherited from the immediate common ancestor.
Figure 3. An example of a phylogeny showing characters by which taxa are
recognised. Characters 1 – 4 are synapomorphies, 5 – 12 are autapomorphies
and 13 is an attribute seen in the salmon and the shark. See text for discussion.
Another way we can think of this is to ask the question “what groups
are specified by what characters?” In Figure 3 given four taxa, of (initially)
unknown interrelationships, then characters 1 and 2 suggest a group Shark +
Salmon + Lizard. Characters 3 and 4 suggest a group Salmon + Lizard. But characters
1, 2, 3 and 4, suggest two nested groups, one more inclusive than the other
((Shark (Salmon, Lizard)).